It is common practice to control a shut-off valve in a fluid line by means of a monitor pilot in combination with a 3-way actuator pilot. For this purpose a control cylinder incorporated in the structure of the shut-off valve has a piston to open the shut-off valve in response to the pressure of control fluid and in the absence of the pressure from the control fluid the shut-off valve closes by spring pressure and/or line pressure. The 3-way actuator pilot normally places the control cylinder in communication with a source of pressurized control fluid to keep the shut-off valve open, but when the actuator pilot is vented through a pilot port it not only cuts off the control fluid from the control cylinder, but also releases control fluid from the control cylinder to cause the shut-off valve to close by means of spring pressure and/or line pressure. The monitor pilot senses changes in the pressure in the fluid line and control the actuator pilot accordingly.
The present invention is directed to improvements in the actuator pilot in such a control system.